Types of Cast Elastomers

• One-Shot Cast Elastomers
• Prepolymer or Hot-Cast Elastomers
• Quasi-Prepolymer Cast Elastomers

One-Shot Cast Elastomers - The majority of all polyurethane products are manufactured using the so-called "one-shot" process. This includes flexible and rigid foams, reaction injection-molded elastomers and solid elastomers. In the one-shot process, the components (raw materials) of the elastomer formulation are metered and mixed simultaneously and then poured into the mold.

One-shot cast elastomers have found wide acceptance due to the ability to employ low-temperature processing, the potential to use lower-cost raw materials (such as polymeric MDI), and the possibility of producing thermoset polyurethanes with very high heat distortion temperatures.

Another advantage is the use of low-pressure gear pumps in processing one-shot cast elastomers. The ratio of the reactants often can be adjusted from approximately 1:1 to 1:2 by volume, which results in more forgiving metering accuracy when compared to hot-cast elastomers with ratios of approximately 9:1.

One-shot polyurethane cast elastomers, based on either liquefied MDI, such as Mondur® PF, Mondur® CD, or liquid PMDI (such as Mondur® MR Light), allow processing at ambient temperature using a combination of polyols and glycols on the B-side at mix ratios of 1:2 or 1:1. At these ratios, slight deviations do not have a dramatic effect on the elastomer's properties, which results in more robust polyurethane systems. The overall properties of the resulting polyurethane elastomers are suitable for many applications, though these properties are not as robust as those produced with the prepolymer or hot-cast elastomer process.

One-shot cast elastomers are found in a wide variety of applications, including gaskets and squeegees, binders for rubber granules in the construction of sports tracks, concrete molds and stamps, joint sealants for concrete, automotive applications, bowling balls, and endcaps for oil and water filters.

One-shot cast elastomers have been particularly successful in electrical encapsulation for cables and transformers. In Europe, these elastomers have almost completely displaced other materials in the encapsulation of power, signal and telephone cables.

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Prepolymer or Hot-Cast Elastomers - The prepolymer process for producing polyurethane cast elastomers is a two-step procedure. Because most prepolymers are solids at ambient temperatures, the chain extension ("vulcanization") is usually conducted at prepolymer temperatures of 80°C (176°F). For this reason, polyurethane elastomers prepared by the prepolymer process are often referred to as "hot-cast" or "hot-cure" elastomers. These high-performance polyurethane elastomers exhibit excellent elasticity combined with high modulus and resilience, low-compression set, and dimensional stability over a wide temperature range

Hot-cast polyurethane elastomers are used in more demanding applications, such as bumper pads, coated conveyor belts, dead-blow hammers, drive belts, forklift tires and wheels, gears, grain and coal chutes, hydraulic seals, mining screens, pipeline pigs, shaft couplers, sprockets, wheel chocks and more.

See the table at the bottom of this page for a summary of properties and applications for hot-cast elastomers.

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• are low viscosity liquids at 25°C
• have favorable mix ratios
• are processable at room temperature or slightly above
• are processable using simple metering equipment
• result in elastomers with excellent overall physical properties

CONTACT US to learn more about quasi-prepolymer cast elastomers.

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